The invention relates generally to a mass flow measurement and control, and more particularly, to a mass flow measurement and control device based on the Coriolis force effect.
Mass flow measurement based on the Coriolis force effect is achieved in the following manner. The Coriolis force results in the effect of a mass moving in an established direction and then being forced to change direction with a vector component normal to the established direction of flow. This can be expressed by the following equation:
            F      ⇀        C    =      2    ⁢          M      ⇀        ×          ω      ⇀      
Where
      F    ⇀    C    (the Coriolis force vector) is the result of the cross product of
  M  ⇀    (the momentum vector of the flowing mass) and
  ω  ⇀    (the angular velocity vector of the rotating coordinate system).
In a rotating system, the angular velocity vector is aligned along the axis of rotation. Using the “Right Hand Rule”, the fingers define the direction of rotation and the thumb, extended, defines the angular velocity vector direction. In the case of the typical Coriolis force flow sensor, a tube, through which fluid flow is to be established, is vibrated. Often the tube is in the shape of one or more loops. The loop shape is such that the mass flow vector is directed in opposite directions at different parts of the loop. The tube loops may, for example, be “U” shaped, rectangular, triangular or “delta” shaped or coiled. In the special case of a straight tube, there are two simultaneous angular velocity vectors that are coincident to the anchor points of the tube while the mass flow vector is in a single direction.
The angular velocity vector changes directions since, in a vibrating system, the direction of rotation changes. The result is that, at any given time, the Coriolis force is acting in opposite directions where the mass flow vectors or the angular velocity vectors are directed in opposite directions. Since the angular velocity vector is constantly changing due to the vibrating system, the Coriolis force is also constantly changing. The result is a dynamic twisting motion being imposed on top of the oscillating motion of the tube. The magnitude of twist is proportional to the mass flow for a given angular velocity.
Mass flow measurement is achieved by measuring the twist in the sensor tube due to the Coriolis force generated by a fluid moving through the sensor tube. Typical known devices use pick off sensors comprising magnet and coil pairs located on the flow tube where the Coriolis force's induced displacement is expected to be greatest. The coil and magnet are mounted on opposing structures, for example, the magnet is mounted on the tube and the coil is mounted on the stationary package wall. The coil will move through the magnet's field, inducing a current in the coil. This current is proportional to the velocity of the magnet relative to the coil.
In low flow applications, however, the tube is relatively small. This makes it difficult or impossible to mount sensing hardware on the tube itself. Prior art solutions to sensing the tube vibrations have been largely unsatisfactory. The present invention addresses shortcomings associated with the prior art.